1. Preheat oven to 375 degrees with rack in middle position.

2. Heat a 10-inch cast iron skillet over medium-high heat. Add 1 tablespoon oil to skillet. When oil shimmers, add asparagus and 1/2 teaspoon salt and cook, tossing occasionally, until asparagus is crisp tender and brown in spots, 3-4 minutes. Squeeze lemon juice over asparagus and transfer to a plate to cool, reserving skillet.

3. Heat 1 tablespoon oil in skillet over medium-high heat. When oil shimmers, add onions and cook, stirring occasionally, until translucent, 3 minutes. Add garlic, spinach, pepper, and 1 teaspoon salt and cook 1 minute until spinach is warmed through.

4. Whisk eggs and milk in a medium bowl until thoroughly combined. Pour into skillet and cook, stirring constantly, just until eggs begin to scramble but are still very wet. Remove from heat. Distribute asparagus in a single layer over eggs, pressing them gently into eggs. Transfer to oven, and bake just until frittata is set, 18-20 minutes. Cool 5 minutes before slicing and serving.

5. Combine capers, parsley, scallions, remaining 1 tablespoon oil, vinegar, and remaining 1/4 teaspoon salt in a small bowl and serve with frittata.